Anti-Spam via SPF: Sender Policy Framework
It is in the best interest of all email users everywhere that domain administrators add SPF records to their domain that indicate what servers are authorized to send email for their domain. Encouraging your domain administrators to adopt SPF protects them from being the victims of spoofing, and reduces the spam threat on not only your server, but others throughout the world as well.
More information can be found at http://www.openspf.org/.